Saturn's moon Iapetus has a gigantic ridge running along its equator that's twice the height of Mount Everest and covers 75% of the moon's equator. And its existence points to something stranger still: Iapetus once had its very own moon.

You can always know immediately when you're looking at an image of Iapetus, because it's the moon that looks like a giant walnut hanging in space. The bizarre equatorial ridge that creates that effect is staggeringly huge: it's a couple thousand kilometers long, and up to 100 kilometers wide and 20 kilometers tall. So where did it come from?

In what's becoming a very popular answer to astronomy mysteries these days, researchers say a giant impact created the ridge. In fact, it's a two-step process - first, another large body would have crashed into Iapetus, throwing up tons of debris which would have then coalesced together into Iapetus's own little sub-moon. It's pretty much the same theory as to how Earth's Moon formed.

However, while the Moon's orbit is such that our satellite is very slowly falling away from us, Iapetus's mini-moon would have slowly fallen into its parent moon. Eventually, gravitational forces from Iapetus would actually start to rip Iapetus Jr. (for lack of a better name) apart, resulting in tons of debris falling towards the surface of Iapetus. Those debris pieces would naturally settle right over the equator, because it takes the least amount of energy for the ridge to form over the planet's rotational bulge.

Planetary scientist William B. McKinnon describes the ancient scene:

"Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet, one after the other, like frozen baseballs. Particles would impact one by one, over and over again on the equatorial line. At first the debris would have made holes to form a groove that eventually filled up."

Iapetus is, relative to Saturn's other major moons, incredibly distant from its planet. That helps give it a far larger gravitational domain than any other satellite in the solar system, allowing it the needed space for such Iapetus Jr. to form together and then come crashing down on its parent moon. No other moon appears to have the necessary space, which explains why Iapetus is the only moon in the solar system with such an unusual ridge.

Fellow researcher Andrew Dombard explains there's still a lot we don't know about the underlying processes, including how long it took for the sub-moon to come together and break apart:

"We're looking at only 100,000 years for a sub-satellite relatively close (to Iapetus) to a billion years for a body that's at the limit of where you could have a stable satellite in orbit around Iapetus. These time scales are certainly plausible considering we have several billion years of time to work with. And longevity is important, because if it happens too fast all geological trace will be lost."

That said, he is confident that this explanation answers all the major questions about Iapetus's origins:

"There are three critical observations that any model for the formation of the ridge has to satisfy. They are: Why the feature is sitting on the equator; why only on the equator, and why only on Iapetus. I think we have something here that explains all those observations. ...We have a lot of corroborating calculations that demonstrate that this is a plausible idea, but we don't yet have any rigorous simulations to show the process in action. Hopefully, that's next."